Interpolating Bathymetric Data
Bathymetry is the underwater equivalent of hypsometry. Where topographic data are often generated by total station, scanner, or photogrammetry,bathymetric readings are frequently made by soundings. Early on soundings were made by means of a rope. Today soundings are frequently made via sonar or other wave based active remote sensing technique. In this lab, we will georeference a map, digitize a series of soundings, and convert these into a DEM. First we willgeoreference the map sheet to the correct coordinate system, then we will project it to Lat-Long WGS84, and check it against some reference data. From there we will create a new theme, digitize a set of soundings, and interpolate a DEM.
- From theclass data archive, obtain one of the five different maps that represent the bathymetry of Lake Titicaca. Obtain also, the lagospri.zip file. You'll need to unzip this and the contents will serve as reference data to check the georeferencing.
- Start a new ArcGIS session and ensure that the layer frame's coordinate system is undefined.
- Add the bathymetric map to the TOC.
- Turn on the Georeferencing toolset.
- Ensure that the bathymetric map is the active layer.
- Click on the Add Control Points tool.
- Plan out what places will form ground control points. In this case, we will use areas where the graticule crosses.
- The map is in DMS and not DD. It will be necessary to convert DMS into DD, the FCC provides a webpage for making this calculation..
- For each GCP, calculate and note the correct DD expression of the coordinate pairs. It can be helpful to sketch this out beforehand.
- Left click on one of the GCPs, then immediately right click and select input X and Y.
- Enter in the correct Lat and Long pair (expressed as DD). Remember that both Lat and Long will have negative signs.
- Repeat the prior two steps for each of the four corners.
- Ensure that the georeferencign transformation is a first order polynomial.
- Make note of the RMS for each of the four GCPs and for the georeferencing solution overall.
- After noting these values, rectify the image. I suggest saving the image as an Erdas Imagine format (*.img) or a geotiff.
- Once the image is georeferenced, define the projection using the define projection tool.
- Start by taking a look at the projection information in the lower right hand corner of the map.
- Inspecting this information may require some translation. Google translate may be helpful.
- Upon inspection, one will find that the data are mercator projected using the La Canoa provisional datum of 1966.
- When defining the projection, use the following: geographic coordinate system>South America>La Canoa.
- Next use the project raster tool to convert the data to WGS84.
- After doing this, use the labospr.shp file to check that the raster is with the instructor to ensure that the the data are properly projected.
- Now we are ready to begin making some bathymetry!
- Use ArcCatalog to create a new point shapefile, during the process of doing so define the projection as Lat Long WGS84.
- Add the new points shapefile into the map along with the lake boundary and the georeferenced and projected bathymetry map.
- Open up the attribute table of the points shapefile and add a field called "depth" and make the field type "double".
- Pick a section of the map, one "square" and start sparking points.
- Start editing, ensure that the points are the active layer, left click to spark a point.
- Use the attributes tool to keep open a dialogue box that can be used for coordinate input.
Here is a quote from the engineers without borders website:
"The 1:25000 topographic map by the Ecuador Geological Survey uses a Transverse Mercator projection, and the Provisional South American Datum of 1956, abbreviated PSAD56, sometimes referred to as La Canoa datum. La Canoa is a town in the jungles of Brazil. To me it looks like a place where the natives feed missionaries to the piranhas, but for some reason they used as the origin for this datum. This datum is used specifically in Ecuador, Venezuela, Peru, and Brazil. Why am I telling you all of this? Well, my buddies who went down on the assessment trip didn’t notice this and had trouble matching their waypoints with the published map. That is why we will use one and only one coordinate system (WGS84 Lat/Long) and vertical datum on this trip."